A DC-DC converter to step up and/or down a direct-current (DC) voltage is known as a voltage converter. The DC-DC-converter is used widely in electric devices including an electric circuit, such as a personal computer, an AV device, a portable telephone, a power system, and etc. Recently, there is an example where the DC-DC converter is used in a power system for a vehicle such as a fuel cell vehicle, electric vehicle, hybrid vehicle, etc.